Increasing temperature increases the rate of both forward and reverse reactions; the forward reaction being exothermic and the reverse being endothermic, as can be seen in the diagram above.
It is to be noted that increasing the energy to match the activation energy means, if the reaction is easily reversible, that the products also have enough energy to become reactants.
Since the above reaction is easily reversible, the endothermic reaction is favored when the temperature is increased. This is because stress applied to the system in the form of increased temperature causes the system to respond with negative feedback (which is the endothermic reaction that absorbs energy).
Take the example of water held in a closed container:
At 35°C, the container would look like this:
The # of molecules vs. temperature graph would look like this:
It is to be noted that molecules constantly change state from liquid to gas and vice versa. However, the number of molecules that change state remain the same, causing the rates to be equal. The system is said to be in equilibrium when the rate if the forward reaction equals that of the reverse reaction.
The energy level diagram would look like this:
When the temperature is increased to 50°C, the rate of the endothermic reaction, which is the forward reaction (liquid to gas), increases and the rate of the inverse exothermic reaction decreases. Therefore, [H2O(g)] increases while [H2O(l)] decreases.
At 50°C, the container would look like this:
The # of molecules vs. temperature graph would look like this:
Eventually equilibrium is reattained at a new value when both forward and reverse reaction have equal rates again.
Le Chatelier's Principle: The equilibrium system responds to a stress with negative feedback until a new equilibrium is reached.
The Haber Process
N2(g) + 3H2(g) ------> 2NH3(g) ΔHo = -92.4 kJ/mol
Energy level diagram:
At STP, [N2] is high, [H2] is high and [NH3] is very low.
The production of ammonia is required.
Stresses that could be applied:
Increased [N2]: Increases the rate of the forward reaction, therefore, produces more NH3.
Increased [H2]: Increases the rate of the forward reaction, therefore, produces more NH3.
Decreased [NH3]: If the product, NH3, is removed as it is formed, more of it is produced as it decreases the rate of the reverse reaction.
Increased pressure: An increase in pressure causes a decrease in volume. The system compensates by decreasing volume, thereby decreasing pressure. Considering that all the reactants and products are gaseous and that there are a lower number of mols of gas (therefore, lower volume) on the products side, the rate of the forward reaction is increased in the system. Thus, more NH3 is produced.
**Temperature: It may seem as though a decrease in temperature would increase the rate of the exothermic reaction (forward reaction in this case). However, the temperature must be high enough in order to achieve the activation energy. The reverse Rx is favoured.
The energy diagram for this reaction:
Increasing temperature increases the rate of both forward and reverse reactions; the forward reaction being exothermic and the reverse being endothermic, as can be seen in the diagram above.
It is to be noted that increasing the energy to match the activation energy means, if the reaction is easily reversible, that the products also have enough energy to become reactants.
Since the above reaction is easily reversible, the endothermic reaction is favored when the temperature is increased. This is because stress applied to the system in the form of increased temperature causes the system to respond with negative feedback (which is the endothermic reaction that absorbs energy).
Take the example of water held in a closed container:
At 35°C, the container would look like this:
The # of molecules vs. temperature graph would look like this:
It is to be noted that molecules constantly change state from liquid to gas and vice versa. However, the number of molecules that change state remain the same, causing the rates to be equal. The system is said to be in equilibrium when the rate if the forward reaction equals that of the reverse reaction.
The energy level diagram would look like this:
When the temperature is increased to 50°C, the rate of the endothermic reaction, which is the forward reaction (liquid to gas), increases and the rate of the inverse exothermic reaction decreases. Therefore, [H2O(g)] increases while [H2O(l)] decreases.
At 50°C, the container would look like this:
The # of molecules vs. temperature graph would look like this:
Eventually equilibrium is reattained at a new value when both forward and reverse reaction have equal rates again.
Le Chatelier's Principle: The equilibrium system responds to a stress with negative feedback until a new equilibrium is reached.
The Haber Process
N2(g) + 3H2(g) ------> 2NH3(g) ΔHo = -92.4 kJ/mol
Energy level diagram:
At STP, [N2] is high, [H2] is high and [NH3] is very low.
The production of ammonia is required.
Stresses that could be applied: